spec for arrow functions adds a runtime check to make sure arrow functions are not instantiated (since they transform into normal functions). spec for template literals wraps all expressions in String rather than simple string concatenation.

// .babelrc{"presets":[["es2015",{"spec":true}]]}

#3659 @kittens added an optional wrapPluginVisitorMethod callback to transform to allow for performance tracking/introspection of plugins. More docs will be added on the website soon.

#3658 sourcemaps will also now have a names field for identifiers to allow debuggers to do re-aliasing of mangled identifiers.

#3518 For spec compilancy, we now will throw on a file with multiple export default.